This invention relates, in general, to a method of and apparatus for testing a telecommunication link and is particularly, but not exclusively, applicable to a method of and apparatus for testing a copper pair (connecting a telephone exchange to a subscriber unit) to determine its ability to support high frequency data transmissions that are ancillary to its originally designated function of supporting voice telephony.
The connection between individual telephone subscribers, whether these are domestic or business subscribers, and a local telephone exchange has traditionally been provided using copper cables consisting of a number of unshielded twisted-pair wires, usually known as xe2x80x9ccopper pairsxe2x80x9d. More explicitly, network topology has a multi-pair cable emanating from an exchange, which multi-pair cable is gradually split out to provide one (and sometimes a plurality) of single copper pairs at a customer""s premises. When these copper pairs were first deployed in local areas, it was assumed that they would be used for transmission of voice signals only; these wireline connections were therefore expected to operate in a frequency range of less than about 4 kilohertz (kHz). Consequently, the planning rules adopted for such wireline (i.e. copper pair) systems were based on easily controlled and measured parameters, such as loop resistance and low frequency attenuation. In the UK, the normal planning limits are 1000 Ohms (xcexa9) loop resistance and seven decibel (7 dB) attenuation at 1 kHz.
These planning limits are achieved by a suitable choice of conductor gauge or diameter depending on the route distance between the exchange and the customer. Longer routes clearly require larger conductors in order to meet the resistance and attenuation limits. Conductors tend to be between 0.3 mm and 0.9 mm in diameter, with increasingly larger diameter conductors being used the further the cable extends from the exchange. This allows for bundles of narrow gauge pairs to be grouped together at an exchange thus minimising cable handling problems.
As the number of new subscribers obtaining telephone services from operators utilising optical feeders increases, telephony providers, whose systems are largely constructed of copper pairs, are increasingly looking to the provision of wideband and broadband services to their customers over their copper pair links. With the advent of wideband and broadband services (such as internet access, video-on-demand and digital data transmissions) as well as increases in the volume of telephony services and traffic, telephony providers are necessarily considering the testing of individual links between exchanges and subscribers in order to ascertain whether or not each link will support the provision of such services. In particular, lines must presently be tested to see if they will support present ISDN services while, as time passes, further tests will more frequently need to ascertain whether or not these twisted pairs will support broadband services requiring technologies such as asynchronous digital subscriber line signalling (ADSL), high speed digital subscriber line signalling (HDSL) and very high speed digital subscriber line signalling (VDSL); these transmission techniques are generically termed xDSL transmissions. Indeed, the more exotic forms of xDSL use wideband techniques for enhanced data capacity (presently up to about ten megabits per second, 10 Mbps), with such wideband techniques distributing information across a number of sub-carriers, e.g. as supported by discrete multitone (DMT) and orthogonal frequency division multiplexing (OFDM).
In contrast with audio signals that have a frequency of less than about 4 kHz, broadband signals may be in the range 25 kHz to 10 MHz, and more usually exceed 1 MHz in order to support broadband applications.
One of the key basic parameters for establishing the suitability of a particular copper pair for carrying such broadband services is its transmission length (arising as a consequence of signal attenuation increasing with transmission length). Unfortunately, this is not readily deducible from the records of a particular operator, even if they are accurate. This is because, although the records show duct routes and section lengths, they do not necessarily indicate how a cable is routed through the duct. For example, it is often found that a copper pair in a specific cable will transverse the full length of the duct to a splice point and then return along the same duct as a pair in another possibly smaller cable.
Furthermore, it will be understood that cabling diameters also play a significant role at high frequencies since signal transmission in cable (at these higher frequencies) is principally through the so-called xe2x80x9cskin effectxe2x80x9d, while loss (i.e. signal attenuation) is also skin effect dependent. Specifically, loss is proportionally greater in smaller gauge cables. While these effects are not important with voice signals, these effects are considerable in relation to higher frequency transmissions because the associated error in the determination can yield misleading results. More particularly, the attenuation at low frequencies (used for narrowband voice communication below about 4 kHz) is primarily dominated by direct current (DC) resistance that is inversely proportional to the cross-sectional area of the wires. At relatively high frequencies (such as employed in xDSL), the dominant skin effect exhibits attenuation that is inversely proportional to the circumference of the wires and also proportional to the square root of the frequency. Thus, the amount of attenuation increases with frequency.
It is equally misleading to use measurements based on grid references in order to predict lengths, because the necessary scaling factor of actual cable length to direct distance is unknown in any specific instance. For example, in the UK, the average scaling factor is probably somewhere between 1.4 and 2.0; the resulting distances are often enough to render a link unsuitable for the provision of wideband or broadband services, while the uncertainty in ascertaining actual cable route lengths makes this method highly inaccurate. In addition, end-to-end connections may include sections of aluminium (or differing numbers of junctions) which will have different transmission characteristics to the copper sections. Aluminium was used in this way when copper prices made copper less economic than aluminium.
Of course, there are other factors that contribute to attenuation or loss, but these are of relatively minor importance and deserve just a passing note, namely loss caused within the dielectric between copper pairs and impedance mismatch at discontinuities, such as at joints between cabling sections.
One alternative approach is for operators to dispatch staff to a customer""s premises to undertake one or a series of measurements of the copper cable and its performance, so called xe2x80x9ctruck rollxe2x80x9d. This is a time consuming and consequently expensive solution, especially if the customer decides not to take the service, or takes it only for a short period.
A variation on full truck roll is for the operator to take field-based sample measurements of cable lengths and performances from the exchange to the local telephone cabinet, from where individual copper pairs are directed to individual subscribers. As the cables from the exchange to the cabinet are shared this would reduce the cost per customer line, but would only give an indication in relation to a few of the copper pairs from any particular exchange. Equally this latter method gives no indication of the length and performance of the copper drops from the cabinet into a customer""s premises. This method is, therefore, again rather inaccurate.
Time domain reflectometry (TDR) is a technique primarily used for determining a discontinuity or breakage in a cable. However, this may be usable to test a link from an exchange to a subscriber unit. Unfortunately, this method is not conducive to copper pairs since there can be many reflections from imperfections, spliced joints, etc., which may mask the ultimate reflection, if any, from the end of the cable. Also, although most telephones are quite well matched to the line in the voice band (when they are on-hook), they may not produce a reflection at a TDR impulse at the point where it may be most useful; in off-hook scenarios telephones are not matched to the line and will have erratic reflection coefficients. Equally, the number of telephones connected at a customer""s premises, and hence the differing impedance produced, could cause spurious results. The main difficulty is that TDR measurements require a fast pulse to operate accurately. This is not possible with copper cables beyond a few hundred metres in length, since the reflected signal will be lost in background noise. Again, therefore, this method is not appropriate.
Line assessment and its suitability for broadband-type services has also envisaged the use of an up-stream test that involves the user directly. Specifically, a potential subscriber who wishes to be considered for a particular broadband services dials a free phone (i.e. toll free) number and is then provided with an automated instruction. Specifically, the instruction requests that the potential subscriber enters a predetermined code, such as #3, on a dual tone multiple frequency (DTMF) keypad, which predetermined tones can then be assessed at receipt by the exchange with respect to data integrity and received signal level, for example. More specifically, an assessment may, in fact, take the form of cross-referencing a look-up table formed from empirical data previously derived from testing known links at audio and broadband frequencies, whereafter the suitability of a specific link under tested is assessed by way of comparison. Alternatively, an algorithm may be provided to project the expected behaviour of the link with broadband signals from the received audio frequency signal.
However, the use of standardised telephony tones emanating from a subscriber terminal is not ideal since such telephony tones are presently only defined within a predetermined but relatively wide tolerance, while the test tones are also (usually) transmitted in the voice band (which is unrelated to the spectrum of interest). Consequently, some error exists in any quantitative assessment at the exchange because of the uncertainty present in the absolute transmitted level, with this basic method also requiring the potential subscriber having access to a DTMF keypad. As such, an absolute level of attenuation caused by the wireline cannot be assessed, especially for high frequencies.
An alternative but similar approach to the one described immediately above obviates the necessity for a DTMF keypad, while also potentially reducing the error associated with the uncertainty in the transmitted level. Specifically, in response to a service query, the operator sends the potential subscriber a dongle that is inserted in-line by the potential subscriber. The dongle has a predetermined tone output that is defined within a much stricter limit than DTMF, and so a subsequent quantification of the level inherently yields a more accurate result of uplink attenuation. Indeed, the dongle need not be restricted to a voiceband transmission and could therefore output a test signal in the frequency spectrum of interest. However, a drawback with this method is that there is still some uncertainty associated with a level of a transmitted test signal.
Uplink measurement techniques are described in UK patent application number 9811984.5 that was filed on Jun. 5, 1998 in the name Northern Telecom Limited, and which UK patent application is further identified by its title xe2x80x9cMETHOD OF AND APPARATUS FOR TESTING A TELECOMMUNICATIONS LINKxe2x80x9d and the first named inventor R. J. Williamson.
Additionally, both of the uplink measurement methods rely upon a potential subscriber independently and directly contacting the operator. Consequently, the operator is entirely reactive to demand since it is must rely upon a direct subscriber action before initially either the test or in sending out a dongle. In other words, the operator is unable to advise all its available clients of the possibility to upgrade their respective services, which immediately inhibits its ability both to encourage subscription to the enhanced service and to optimise its revenue. Ideally, operators generally prefer to be proactive, and therefore to approach existing clients with a offer to install the enhanced service that is already available by virtue of the wireline communication resource being of a standard sufficiently high to support an enhanced service, such as broadband xDSL data.
Notwithstanding the foregoing techniques, it is generally considered that, to date, an accurate assessment of a loop for broadband and wideband applications, such as ADSL and VDSL, can only be obtained using attenuation measurements at the relatively high frequencies used for these services. Moreover, in the case of an ADSL scheme, it is considered necessary to measure the loop over its full reach, namely from the exchange to the customer""s premises. For VDSL, which use higher frequencies, present techniques only support a loop reach of about one kilometre (1 km), and so measurements are considered necessary on a street cabinet to customer premises basis.
A requirement therefore exists for a method and apparatus of testing a wireline communication resource that can address at least some of the deficiencies in the prior art, which method preferably can be employed to assess a number of alternative enhanced data services.
According to a first aspect of the present invention there is provided a method of assessing suitability of a wireline communication resource for supporting data transmissions at relatively high frequency between an infrastructure node and a subscriber terminal, the wireline communication resource inherently arranged to support voice band transmissions at relatively low frequency, the method comprising the steps of: at a test point, sending a test signal at the relatively high frequency to the subscriber terminal, the test signal having a predetermined level; detecting an attenuated level of the test signal at the subscriber terminal; communicating an indication of the attenuated level of the test signal to the test point; and at the test point, assessing the suitability of the wireline communication resource for supporting data transmission at relatively high frequency based upon receipt of the indication of the attenuated level and the predetermined level.
In a preferred embodiment, the method further comprises the step of: communicating the indication of the attenuated level to the test point further comprises the step of regulating a current of the wireline communication resource in response to the current generated to be representative of the attenuated level; and the step of assessing further comprises the step of detecting variations in the current through the wireline communication resource to ascertain the level of attenuation.
A DTMF chip can be used to provide a mechanism for generating tones in response to the current representative of the attenuated level, and further to regulate a voltage supplied to a voltage regulator of an associated test circuit with the tones to vary the current drawn by the test circuit from the wireline communication resource.
Alternatively, tone indicative of the attenuated level can be modulated onto the wireline communication resource for detection at the test point.
A further alternative embodiment forms a voice band signal in a non-linear circuit in response to the receipt of the test signal. The voice band signal has an amplitude indicative of a level of attenuation caused to the test signal by the wireline communication resource, and so subsequent modulation of the voice band signal onto the wireline communication resource communicates the attenuation to the test point.
In another aspect of the present invention there is provided a wireline test system for assessing an ability of a wireline communication resource to support data transmissions at relatively high frequencies between an infrastructure node and a subscriber terminal, the wireline communication resource inherently arranged to support voice band transmissions at relatively low frequency, the wireline test system comprising: a) a test desk arranged both to co-ordinate a transmission of relatively high frequency test signals of known level down the wireline communication resource and to assess a level of attenuation caused to the test signal in response to a report signal received subsequently thereby; and b) subscriber associated test circuitry for coupling to the wireline communication resource, the subscriber associated test circuitry comprising: a detector, responsive to the relatively high frequency test signals, for generating a signal representative of an attenuated level of the relatively high frequency test signals through the wireline communication resource; and means for communicating an indication of the attenuated level of the test signal to the test desk.
In a third aspect of the present invention there is provided a subscriber associated test circuitry for coupling to a wireline communication resource inherently arranged to support voice band transmissions at relatively low frequency, the subscriber associated test circuitry further arranged to receive relatively high frequency test signals of known level transmitted to the subscriber associated test circuitry through wireline communication resource, the subscriber associated test circuitry comprising: a detector, responsive to the relatively high frequency test signals, for generating a signal representative of a level of-the relatively high frequency test signals through the wireline communication resource; and means for communicating an indication of the level of attenuation of the test signal to the test desk.
The preferred embodiments of the present invention advantageously provide an accurate method and apparatus for testing a telecommunications link between an exchange and a subscriber, which method is especially applicable in relation to an operator being able to assess the suitability of the telecommunication link for high frequency, broadband-type systems. Beneficially, the method of testing telecommunications link according to the preferred embodiment of the present invention does not require an engineer to visit a subscriber.
The relatively simple circuitry employed in a detector can be easily incorporated into a subscriber terminal, while the exchange provides an accurately controlled test signal that is better suited to making an accurate assessment of wireline properties, especially in relation to the possibility for xDSL communication.
The methods of line assessment of the various aspects of the present invention can be used for different kinds of wireline communication resources, including coaxial systems (e.g. for cable television), twisted pairs and other transmission systems whose attenuation is frequency dependent.